Stromal progenitors (SP) are cell populations that are capable of proliferation and differentiation, and also provide support for surrounding tissues and cells.
SP are rare cell elements located in the tissues of living beings, which are designed to perpetuate their function by turnover of damaged and/or senescent cells.
SP were initially found in high-turnover tissues, such as the hematopoietic and epithelial system, but can be found, although less frequently, in tissues and organs having little or no regenerative capacity, such as the central nervous system.
Originally, bone marrow studies started more than thirty years ago were able to identify certain SP, which will be referred to hereinafter as mesenchymal SP, which are capable of maintaining hemopoiesis and osteogenesis, thereby providing functional and structural support.
In vitro studies showed that these cells have a high proliferative potential, as well as differentiating capacities, in certain appropriate conditions, that is, the ability of converting into cell elements belonging to bone, cartilage, adipose, muscular and nervous tissue.
This knowledge related to the characteristics of bone marrow mesenchymal SP have extended the scope of research to introduce novel therapies in the so-called regenerative medicine.
Due to their differentiating potential, bone marrow mesenchymal SP have been studied for regeneration of injured tissues after trauma and acute and chronic degenerative events, such as cardiopathies. In oncology, they may be used to carry drugs having an antitumor action and also find application in autoimmune diseases, due to the production of immune response modulating molecules. Furthermore, due to their support function, they have been found to be useful as an aid in hemopoietic stem cell transplantation.
Some of these studies led to clinical applications of SP, in areas such as myocardial infarct, diabetes mellitus, autoimmune diseases, bone regeneration, burns, lipodystrophies and liver failure.
In all the above situations, and for an effective therapeutic application, a great number of SP needs to be infused or transplanted, i.e. in the order of various millions per kilogram weight of a patient.
This requires a prolonged expansion of SP outside of the donor, i.e. in vitro in culture flasks. This is required because of the poor presence of SP in the original bone marrow tissue, and may be a limitation.
Furthermore, the collection site (the bone marrow) may not be easily accessible and be damaged due to the presence of neoplastic cells or simultaneous pharmacological treatments.
Therefore, further SP collection sites have been considered, such as the periosteum, bone trabeculae, the skeletal muscle, the lung, the umbilical cord and particularly the subcutaneous adipose tissue (AT).
Concerning the AT, the main cell is the adipocyte, which is as large as about 100 μm and fulfills the main role of the AT, i.e. storing energy in the form of triglycerides introduced by diet.
Also in the AT, the SP are a pool of progenitors which replicate in response to appropriate hormone stimulation, thereby allowing a part of the progenies to differentiate into mature adipocytes, and also act as a support to vascular structures, whereby they are defined as stromal pericytes.
Adipose tissue is distributed in many anatomical districts into the body and divided into two fat types: white adipose tissue and brown adipose tissue.
Brown adipose tissue is scarcely traced in newborn and adults, in particular it can be found in rare anatomical regions, primarily into the interscapular region. White adipose tissue is the major component of adipose tissue of adults and is subdivided into two groups according to the resident anatomical district. Subcutaneous adipose tissue (SAT) makes up 80% of all body adipose tissue and is distributed mainly in the abdominal region, buttocks and flanks. Visceral white adipose tissue (VAT) represents about 10% of all body adipose tissue and is present in the omental and mesenteric regions (Lee M J et al, Adipose tissue heterogeneity: implication of depot differences in adipose tissue for obesity complicationsm Mol Aspects Med 2013;34:1-11).
Another subcategory of visceral adipose tissue is the epicardial adipose tissue (EAT) which is the fat adjacent to the coronary arteries and myocardium and is actually considered to be brown adipose tissue because of its functional properties (Salgado-Somoza A. et al, Proteomic analysis of epicardial and subcutaneous adipose tissue reveals differences in proteins involved in oxidative stress, Am J Physiol Heart Circ Physiol 2010; 299:H202-H209).
AT mass in adult humans is a function of diet and life style and ranges from 2-3% the total weight in an athlete up to 60-70% in an obese individual.
The increased occurrence of obesity has increased AT availability, also due to an increase in cosmetic surgery for reducing subcutaneous adipose mass for aesthetic purposes or else. In these cases AT collection may presently be performed by liposuction.
As a result, AT is a potential source of SP, due to its abundance and accessibility.
Adipose depots of VAT and SAT, apparently morphologically similar, display instead numerous intrinsic differences in the adipocyte progenitor population, defined previously as stromal progenitors (SP). These two stromal progenitor populations have intrinsic differences in genomic expression profile, multidifferentiative capability, cellular response to both genetical factors and microenvironment (Macotela Y et al, Intrinsic differences in adipocyte precursor cells from different white fat depots, Diabetes 2012; 61:1691-1699; Peinado J R et al, The stromal vascular fraction of adipose tissue contributes to major differences between subcutaneous and visceral fat depots, Proteomics 2010; 10: 3356-3366).
SAT has a higher number of stromal progenitors comparing with VAT (Lee M J et al, supra) with a higher proliferative and differentiative capability versus VAT (Ong W K et al, Identification of specific cell-suface markers of adipose-derived stem cells from subcutaneous and visceral fat depots, Stem Cell Reports 2014;2:171-179).
Zuk P A et al, in the article “Human adipose tissue is a source of multipotent stem cells”, published in Molecular Biology of The Cell (2002), started various studies to assess the analogy between SP of adipose and bone marrow origin. These initial tests showed a number of analogies in terms of differentiation potential and antigen expression, and this preliminary data allowed the introduction of SP in many fields of regenerative medicine for cardiology and cosmetic medicine applications, particularly starting from patient-derived, i.e. autologous cells.
The present state of the art is limited by the large amounts of subcutaneous AT that must be collected to obtain an adequate number of SP. These volumes of collected fat are usually above 0.5 L and may be as large as 1 L. While these are large volumes in absolute terms, they have relatively little incidence on an obese or overweight patient, i.e. having a Body Mass Index (BMI) parameter exceeding 25. Those volumes cannot be obtained from low BMI individuals (having a BMI of less than 18.5), who may hardly have the required amount of autologous SP.
Therefore, this method in the prior art is only applicable to a limited number of patients.
Moreover, invasive surgical procedures are required, involving the hazards of any surgery, particularly fat embolism.
Further, AT collections always require a general anesthesia of the patient, with the related requited care and potential hazards.